Correct - no timing issue.

And It is AND (not OR). Always switch both signals into ON state.

At any given time only one of them will be used (but that will be depending on the specific hardware)  and there is no disadvantage to switch both signals. 

From a timing prospective - there are no limitations and it is not important which one goes into "ON" state first and which is second.

I would recommend also adding a 0.1sec delay after power has been turned on (so that voltage can stabilize) before any commands to the bitfury chips are sent.